Boundary layer turbulence and flow structure over a fringing coral reef

Abstract

Measurements of velocity and rates of turbulence were made across a fringing coral reef in the Gulf of Aqaba, Red Sea, to determine the effect that the rough topography has on boundary layer mixing and flow dynamics. Observations were made at two fore-reef sites and a nearby sandy slope. The friction velocity, u * , and drag coefficient, C D , were determined directly from turbulent Reynolds stresses measured using acoustic Doppler velocimeters. Values of C D for the coral substrates ranged from 0.009 to 0.015, three to five times greater than over the sandy bottom site. The turbulence dissipation rate, e, was determined by fitting spectra of vertical velocity to the theoretical ''5/3'' law expected for the inertial subrange of turbulence. There was a local balance between production and dissipation of turbulent kinetic energy, signifying that we could estimate u * from either the mean velocity profile, turbulence, or dissipation rate of turbulent kinetic energy. Estimates from all three measures agreed well with mean u * /U o ranging from 0.10 6 0.03 to 0.12 6 0.03, indicating that existing turbulent boundary layer flow theory can be applied to flows over the rough topography of coral reefs. The bottom topography, by enhancing both reef scale and local drag and mixing levels, allows reef biota to more effectively exchange dissolved and particulate matter with oceanic waters.

The Modification of Bottom Boundary Layer Turbulence and Mixing by Internal Waves Shoaling on a Barrier Reef

“…The inclusion of B in the TKE budget does not significantly improve the balance between turbulent production and sink terms. Our findings differ from observations of boundary layer turbulence over a fringing coral reef in the Gulf of Aqaba by Reidenbach et al (2006), who found a local balance between P and « at locations 0.1, 0.3, and 1 mab.…”

“…The line was constrained to go through the origin so that u * 5 0 when U 1m 5 0. C D was estimated to be 0.017 6 0.001, which is much larger than the canonical value for sand and mud bottoms (0.0025; Gross and Nowell 1983), but comparable to values of C D found using similar methods (0.009-0.015) over coral substrates in Eilat, Israel, by Reidenbach et al (2006).…”

Field observations of wave‐driven circulation over spur and groove formations on a coral reef

“…The total kinetic energy due to turbulence (TKE) is TKE5 1 2 u 0 2 1v 0 2 1w 0 2 . Mean bottom stress, s b , was computed from the turbulent Reynolds stress, which are assumed constant within the inertial sublayer, [e.g., Reidenbach et al, 2006],…”

“…The C D was similar O(0.01) at both sites in the cross-shore direction and in the alongshore direction over the spur (A3), while it was much larger O(0.1) in the alongshore direction over the groove (A2). Fits to logarithmic profiles were performed for the alongshore Eulerian velocity profiles, [e.g., Reidenbach et al, 2006] but the results for bottom roughness scale z 0 (A3: 0.009 6 0.023 m, A4: 0.008 6 0.016 m, C1: 0.02 6 0.04 m) and offset height d (A3: 0.62 6 0.12 m, A4: 0.51 6 0.33 m, C1: 0.57 6 1.36) had very large scatter. Use of Grant and Madsen [1979] to remove the effect of waves and predict a physical roughness scale k N (A3: 0.06 6 0.03 m, A4: 0.05 6 0.04 m, C1: 0.12 6 0.12 m) also had very high scatter.…”

Near-surface enrichment of zooplankton over a shallow back reef: implications for coral reef food webs

“…Turbulence is greater near the bottom on reefs (Reidenbach et al 2006). Although larval responses to turbulence are poorly known, crab megalopae can ascend in a laboratory tank when turbulence increases and descend when it decreases (Welch et al 1999).…”